Insulated large-diameter or -caliber plastic pipe is made by first extruding a rectangular-section thermoplastic-resin profile or tube. This tube is then wound up on a mandrel in laterally tightly engaging turns that are welded together where they contact each other laterally to form a mainly hollow wall.
Basically, any desired thermoplastic suitable for the manufacture of tubes ca be used for the plastic from which the hollow plastic profile is made. More particularly, the plastic may be a polyolefin such as polyethylene (PE) or polypropylene (PP). The plastic may have an integrated reinforcement of fibers and be provided with finely divided fillers, more particularly, mineral fillers as described in above-cited application 07/583,376.
British patent 1,438,322 describes the formation of pipe from a hollow plastic profile in the form of a single-cell profile of square or rectangular external cross-section. The hollow plastic profile is extruded with a substantially round internal cross-section. A cooling fluid is introduced into this hollow plastic profile during extrusion and is passed through at least one turn. The turns are welded in contiguous relationship at an appropriate temperature and with an appropriate axial pressure.
It has been found that flanges with overlapping weld seams or a positive lock of the turns can be dispensed with in the manufacture of tubes, and particularly large-caliber pipes, made from hollow plastic profiles. The known steps utilize the fact that a hollow plastic profile at deformation temperature can without difficulty be coiled with contiguous turns. Axial pressures required to weld the adjacent contact walls are then produced preferably kinematically. At the same time, a support core produced in the hollow plastic profile by the introduction of cooling fluid acts to prevent collapse of the profile. This allows adequate compression and upsetting in the axial direction for a perfect welding of the surfaces of the adjacent contact walls of the helically coiled hollow plastic profile.
In the known steps, it is possible to adjust conditions so that the axial pressure produces a permanent deformation and changes the cross-section of the hollow plastic profile on welding. What occurs depends on the value to which the axial pressure, i.e. the upsetting or compression, is set and on the value to which the temperatures are set, and also on the degree of cooling after extrusion.
The known steps have proved satisfactory, but result in plastic pipes of relatively considerable weight for a predetermined design stress. They can be manufactured only with hollow plastic profiles whose rectangular cross-sectional shape differs only little from a square cross-sectional shape.